The present invention is generally related to a relay apparatus equipped with a function capable of canceling interference from a transmission antenna to a reception antenna, that is so called a loop operations of signals between a reception antenna and a transmission antenna. More specifically, the present invention is directed to such a relay apparatus capable of firmly removing loop waves contained in received input signals by correcting amplitude errors and phase errors.
The Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) system is directed to such a TV system which has been developed in order to provide the HDTV (High Definition Television) broadcasting services for fixed receiver sites, and also the broadcasting services for the mobile receiver sites, while utilizing the frequency band of the presently-operated ground-based analog television broadcasting system, such as VHS or UHF. This ISDB-T system employs the OFDM (Orthogonal Frequency Division Multiplexing) system as the transmission system. This OFDM system owns the following various merits. That is, not only this OFDM system can be hardly adversely influenced by multipath interference, but also the modulation method can be changed every sub-carrier, and furthermore, the SFN (Single Frequency Network) can be established. In this case, this SFN corresponds to such a system that the same program can be relayed/transferred in the same frequency (channel). Since the SFN can provide the broadcasting services with employment of the narrow-band frequency, the frequency resource can be effectively utilized. Since very large numbers of the ground-based analog television broadcasting channels have already been utilized, there are many areas where continuous and broad frequency bands cannot be newly secured. Under such difficult conditions, in order to conduct the above-described ISDB-T system, it is preferable to establish such a broadcasting system with employment of the SFN, since this SFN broadcasting system may be practically operated in connection with the presently available ground-based analog television broadcasting system.
However, in the case that the broadcasting system using the SFN is established, there is a problem such as a so-called “loop phenomenon”, since broadcasting signals are transmitted/received by using the same frequency in relay stations. In this loop phenomenon, broadcasting signals having the same frequency which are transmitted from a transmission antenna are returned to a reception antenna in a loop manner. As a consequence, this loop phenomenon may not only cause signal transfer qualities to be deteriorated, but also may induce oscillations in such a case that coupling amounts between transmission antennas and reception antennas are large, so that retransmissions of broadcasting signals cannot be carried out.
To solve such a problem, Japanese Patent Publication No. Hei. 11-355160 has proposed the loop canceller capable of canceling the signal loop occurred between the transmission antenna and the reception antenna of the relay apparatus operated in the SFN. FIG. 6 shows a structural diagram of this conventional relay apparatus. In the relay apparatus of this conventional loop canceller, based upon the reference signal 70 which is derived from the input side of the relay broadcasting unit 400 and then divided from the divider 300, the digital signal processing unit 902 employed in the loop canceller 900 produces such a duplicated loop signal having the transmission characteristic equal to the transmission characteristic of the loop transfer system established between the transmission antenna 500 and the reception antenna 100 provided in the relay apparatus. Then, the subtracter 907 subtracts the duplicated loop signal which is produced in the digital signal processing unit 902 from the received input signal 50 of the relay apparatus. This received input signal 50 contains the loop signal derived from the transmission side. As a result, the signal loop operations occurred between the transmission antenna 500 and the reception antenna 100 may be canceled.
However, in the above-explained conventional relay apparatus, while the digital signal processing unit 902 converts the analog signal into the digital signal and thereafter executes the signal process operation, the quantization error may occur when the quantizing operation of this analog-to-digital conversion is carried out. For instance, in a 4-bit A/D converter, precision of resolution becomes 24=16. In the case that numeral values defined from 0 to 1 are expressed by 4 bits, these numerals cannot be expressed only by an integral multiple of 1/16=0.0625. As a result, when such a numeral value of 0.07 is repressed by 4 bits, this numeral value is expressed by either 0.0625 or 0.125 (=0.0625×2), so that such an error of either 0.075 or 0.055 may be produced with respect to 0.07.
Since there are such a quantization error caused by the quantizing operation of the analog-to-digital conversion by the digital signal processing unit 902, the delays caused by the signal processing operation, and other factors, the duplicated loop signal which is produced by the digital signal processing unit 902 contains either the amplitude error or the phase error. Thus, since both the amplitude and the phase of the loop signal derived from the transmission antenna 500 cannot be completely made coincident with both the amplitude and the phase of the duplicated loop signal, the loop wave 40 derived from the received input signal 50 cannot be firmly removed, so that the problem such as the deteriorations in the signal transmission qualities could not be completely solved.